Cellular operators have started to offer mobile broadband based on Wideband Code Division Multiple Access (WCDMA)/High Speed Packet Access (HSPA). Further, end user performance requirements are steadily increasing due, in part, to resource demands of data applications. The increase of mobile broadband users has resulted in heavy traffic volumes, and the demands placed on HSPA networks to handle such users have grown significantly. Therefore, techniques that allow cellular operators to manage their spectrum resources more efficiency are of great importance. However, since improvements in spectral efficiency per link are approaching theoretical limits, the next generation technology seeks to improve spectral efficiency per unit area. In other words, the next generation technology seeks to provide a uniform user experience to users anywhere inside a cell by changing the topology of traditional networks.
In this regard, the Third Generation Partnership Project (3GPP) is currently working on heterogeneous networks, as described in Ericsson et al., “R1-124512: Initial considerations on Heterogeneous Networks for UMTS,” 3GPP, TSG RAN WG1 Meeting #70bis, Oct. 8-12, 2012, San Diego, Calif.; Ericsson et al., “R1-124513: Heterogeneous Network Deployment Scenarios,” 3GPP, TSG-RAN WG1 #70bis, Oct. 8-12, 2012, San Diego, Calif.; and Huawei et al., “RP-121436: Proposed SID: Study on UMTS Heterogeneous Networks,” 3GPP, TSG RAN Meeting #57, Sep. 4-7, 2012, Chicago, Ill. Traditional cellular communications networks are homogeneous networks. A homogeneous network is a network of base stations (e.g., Node Bs) in a planned single-layer layout in which all base stations have similar, or the same, transmit power levels, antenna patterns, receiver noise floors, and backhaul connectivity to the data, or core, network. Moreover, all base stations offer unrestricted access to user terminals in the network, and serve roughly the same number of user terminals. Some examples of cellular communications networks that traditionally have utilized homogeneous network layouts include, for example, Global System for Mobile communications (GSM) networks, WCDMA networks, High Speed Downlink Packet Access (HSDPA) networks, Long Term Evolution (LTE) networks, WiMax networks, etc.
In contrast, a heterogeneous network includes a number of macro, or high-power, base stations in a planned layout and a number of low-power base stations. One example of a heterogeneous network 10 is illustrated in FIG. 1. In FIG. 1, only one macro cell 12 is illustrated. However, the heterogeneous network 10 typically includes many macro cells 12. As illustrated, the heterogeneous network 10 includes a macro, or high-power, base station 14 serving the macro cell 12 and many low-power base stations 16 serving corresponding small cells 18. The low-power base stations 16 may include, e.g., micro base stations, pico base stations, femto base stations, and/or relay base stations. The transmit power of the low-power base stations 16 is relatively small as compared to that of the macro base station 14. For example, in some implementations, the transmit power of the low-power base stations 16 may be up to 2 Watts, whereas the transmit power of the macro base station 14 may be up to 40 Watts. The low-power base stations 16 are deployed to eliminate coverage holes in the macro layer (i.e., the layer of macro base stations 14), mitigate the shadow fading effect, and improve the capacity in traffic hot-spots. Due to their low transmit power and smaller physical size, the low-power base stations 16 can offer flexible site acquisitions.
Heterogeneous networks can be divided into two categories. In a first category, each of the low-power base stations 16 has a different layer 3 (L3) cell Identifier (ID) (and a different scrambling code), and the L3 cell IDs of the low-power base stations 16 are different than the L3 cell ID of the macro base station 14. One example of this first category is illustrated in FIG. 2. As shown in FIG. 2, the macro base station 14 and the low-power base stations 16 create different cells (Cell A, Cell B, and Cell C) having different cell IDs. In this case, the low-power base stations 16 provide load-balancing and, as a result, huge gains in system throughput and cell edge user throughput can be achieved. One disadvantage of this approach is that, since each low-power base station 16 creates a different cell, a user terminal (e.g., a User Equipment device (UE)) needs to do a soft handover when moving from the macro cell 12 to one of the small cells 18 or when moving from one small cell 18 to another small cell 18. Therefore, higher layer signaling is needed to perform these soft handovers.
In a second category, all of the low-power base stations 16 have the same L3 cell ID as the macro base station 14. In this category, the aggregate of the macro cell 12 and the small cells 18 is referred to as a combined cell, a soft cell, or a shared cell. As such, this second category is referred to as a combined cell deployment, a soft cell deployment, or a shared cell deployment. The terms “combined cell” and “combined cell deployment” are used herein. One example of a combined cell deployment of the heterogeneous network 10 is illustrated in FIG. 3. As shown in FIG. 3, the macro base station 14 and the low-power base stations 16 share the same cell ID such that, together, the macro base station 14 and the low-power base stations 16 serve a single combined cell (Cell A). The combined cell avoids the need for frequent soft handovers and the corresponding higher layer signaling.
A combined cell deployment typically uses one of two transmission modes, namely, a Single Frequency Network (SFN) transmission mode and a spatial reuse transmission mode. In the SFN transmission mode, all nodes (i.e., the macro base station 14 and the low-power base stations 16) transmit the same pilot channel, data, and control information. In this case, only one user terminal (which is some cases is referred to as a UE) can be served from all nodes at any time. Hence, the SFN transmission mode is useful for coverage improvement. Furthermore, the SFN transmission mode supports legacy user terminals (e.g., user terminals that do not support the spatial reuse transmission mode). FIG. 4 is a graphical illustration of the SFN transmission mode for one example of a combined cell in a HSPA network.
In the spatial reuse transmission mode, all nodes (i.e., the macro base station 14 and the low-power base stations 16) transmit the same pilot channel, but data and control information transmitted from one node is different from that transmitted from every other node, or at least one other node. In other words, one node will transmit data and control information for one user terminal while, at the same time, another node will transmit data and control information for another user terminal. In this manner, resources (e.g., spreading codes or channelization codes, scrambling codes, frequencies, etc.) can be spatially reused. The spatial reuse transmission mode provides load balancing gains and, as a result, the capacity of the combined cell can be significantly increased. FIG. 5 is a graphical illustration of the spatial reuse transmission mode for one example of a combined cell in a HSPA network.
In a combined cell deployment, the SFN transmission mode causes wastage of resources and does not provide capacity benefits when the load of the combined cell is high. The capacity of the combined cell can be increased using the spatial reuse transmission mode. However, only spatial reuse gains can be obtained in the spatial reuse transmission mode. As such, there is a need for systems and methods for increasing capacity (i.e., improving spectral efficiency) in a combined cell deployment of a heterogeneous network.